The present invention relates to a composite material the surface of which is made to be hydrophilic or antifouling by forming a film layer containing a photocatalytic substance on a material surface of a substrate, a lens, a glass plate, a mirror, etc, improving the hydrophilicity or antifouling property of the material surface.
A technique, which keeps a surface of a material clean by forming a photocatalytic substance layer on a material surface, has been known. This technique enables removing dirt easily by decomposing dirt adhering to the material surface using a photocatalytic action of a photocatalytic substance layer provided on the material surface and by rinsing the material surface with water or the like.
As a composite material employing a material surface cleaning technique using such a photocatalytic substance layer, there was one which was proposed by Japanese Patent Laid-open No.H10-36144 applied for by this applicant. Its structure is described, referring to cross-sectional drawing FIG. 2. In this, a surface is made hydrophilic by providing a SiO2 film 6 porously on the surface of a base material 2. A surface of a SiO2 film 6 is originally hydrophilic, the hydrophilicity, however, is improved by making the surface porous. With a SiO2 film 6 exclusively provided, hydrophilicity decreases with dirt adhering to the surface if it is exposed to the air for a long period of time. Accordingly, by providing a TiO2 film 4 between a SiO2 film 6 and a base material 2, the photocatalytic decomposition action of the TiO2 4 film is used to decompose dirt adhering to the surface of a SiO2 film 6 and to keep the surface of a SiO2 film 6 clean, maintaining the hydrophilicity.
FIG. 3 shows a graph indicating spectral reflectance characteristics in a visible region (380xcx9c780 nm) in a conventional structure shown in FIG. 2 with a SiO2 film thickness made at 10 nm and when a film thickness of a TiO2 film 4 changes to 50, 100, 150, and 200 nm. FIG. 4 shows a graph indicating spectral reflectance characteristics with like samples. As shown in FIG. 3 and FIG. 4, when a film thickness of a TiO2 film 4 is 50 nm, relatively flat reflectance characteristics and transmittance characteristics were obtained, while the amplitude in reflectance increases with a film thickness of 100, 150, and 200 nm as wavelengths change.
Additionally, Table 1 shows initial contact angles and contact angles after being exposed to black light with an intensity of 1 mW/cm2 for four hours when oil is applied onto each material surface of a TiO2 film 4, using samples prepared in a conventional structure shown in FIG. 2 with a film thickness of 50, 100 or 150 nm similar to FIG. 3 and FIG. 4 respectively.
As can be seen from the results of Table 1, for a hydrophilic material using the above-stated photocatalytic substance, a photocatalytic substance film 4 with a film thickness of 100 nm or more is required to obtain a photocatalytic decomposition action to maintain hydrophilicity. As shown in FIG. 3, however, when a film thickness of a TiO2 film 4 is 100, 150 or 200 nm, a coloring action takes place on a material surface due to increased amplitude occurring in spectral characteristics. This is because optical interference in a visible light region occurs at a TiO2 film 4 due to formation of a thick TiO2 film 4. Thus, if the amplitude in spectral characteristics is large, the color tone of an original material surface is spoiled. Additionally, to provide a thick film thickness, more photocatalytic substance material is required.
The present invention has been realized in consideration of the above-mentioned points, to provide a composite material which can obtain sufficient antifouling properties or hydrophilicity even when a photocatalytic substance film is thinner than a conventional film. Thus an optical interference action in a photocatalytic substance is suppressed and an amount of a photocatalytic substance to be used is decreased.
The present invention possesses a base material, a transparent metal thin film provided on the surface of the base material, and a hydrophilic functional layer containing a photocatalytic substance provided on a surface of the transparent metal thin film. According to experiments conducted by inventors, it was found that, by providing a transparent metal thin film, a photocatalytic property improves. Due to this, because hydrophilicity or antifouling can be obtained even if a hydrophilic functional layer containing a photocatalytic substance is thinner than conventional layers, an amount of a photocatalytic substance to be used can be reduced. Additionally, by making a hydrophilic functional layer containing a photocatalytic substance thinner, optical interference in a hydrophilic functional layer containing the photocatalytic substance can be reduced and the color tone of a base material can be maintained.